Birmingham particle physicists are pioneering efforts to explain how matter acquires its mass

Like most great discoveries, the July 2012 observation of a new particle consistent with the long-sought Higgs boson posed almost as many questions as it answered.

This discovery by the ATLAS and CMS experiments at the CERN Large Hadron Collider (LHC), where Birmingham physicists have significant involvement and leadership, marked a breakthrough in our understanding of the fundamental laws of Nature. However, the discovery was just the first step: the experimental focus promptly shifted to the measurement of the particle’s properties, notably its interactions with other particles and searches for additional Higgs bosons, as predicted by many physics scenarios beyond the ‘Standard Model’ (SM) that successfully describes all of the currently known particles and their interactions.

Birmingham particle physicist Dr Kostas Nikolopoulos was coordinator of the ATLAS H→ZZ group, one of the main channels that identified the Higgs boson, during the time of the discovery.

‘Although the observation of a Higgs boson completes the SM of particle physics, to unambiguously characterise this newly observed particle, detailed studies of its properties (mass, production rates, and quantum mechanical properties) are needed. At the same time, searches for other additional Higgs bosons are important,’ he explains.

Following the discovery, detailed studies to characterise the observed particle, and searches for additional Higgs bosons, were launched. Kostas, a Lecturer and Birmingham Fellow from the School of Physics and Astronomy, is now pioneering a new field of study.

‘The Higgs field was introduced in the 1960s by the Englert-Brout-Higgs mechanism to complete the formulation of the SM of particle physics by explaining how the W and Z bosons obtain their mass,’ he says. ‘The Higgs boson is a massive elementary particle that appears in the simplest version of the mechanism. However, the masses of the matter particles – the quarks and the leptons – are generated in the SM through ad-hoc couplings to the Higgs field. To date, there is little evidence that this is the case for the lighter matter particles.’

Kostas was recently awarded the College’s ‘Paper of the Month’ award for the article ‘Search for Higgs and Z Boson decays to φγ with the ATLAS Detector’, which was published in Physical Review Letters. This is a publication by the ATLAS collaboration, with strong involvement from Kostas and the Birmingham group.

‘We are trying to directly probe the interactions of the Higgs boson to the light quarks for the first time. This is a new methodology, and the means to collect the data needed to be developed. Already these results offer new information, and will become more stringent as we improve the analysis and add more data.

Such decays of the Higgs boson would be very rare, but if observed, would tell us something very fundamental about how matter acquires its mass.’

Kostas, who was recently awarded a prestigious Starting Grant of 1.5 million euros from the European Research Council to put together a dedicated team to perform and expand these studies within the ATLAS collaboration over the next five years, says the study has a long way to go: the more data the LHC delivers, the more scientists will learn – and the more likely they are to discover new physics.

But he adds: ‘There’s a real possibility that new physics is hiding there, as we are directly probing this region for the first time.’

More information about the activities of the ATLAS Particle Physics group in exploring the Higgs sector can be found in the dedicated web-page.